Investigation on the oxidation behaviour of gamma titanium aluminides coated with thermal barrier coatings

Abstract

In the present study, the applicability of thermal barrier coatings (TBCs) on g-TiAl alloys was investigated. Two alloys with the chemical compositions of Ti-45Al-8Nb-0.2B-0.15C and Ti-45Al-1Cr-6Nb-0.4W-0.2B-0.5C-0.2Si were used. Before TBC deposition, the specimens were pre-oxidised in laboratory air or low partial pressure oxygen atmosphere. Yttria partially stabilised zirconia top coats were then deposited using electron-beam physical vapour deposition (EB-PVD). The oxidation behaviour of the g-TiAl specimens with TBC was studied by cyclic oxidation testing in air at 850 and 900 8C. Post-oxidation analysis of the coating systems was performed using scanning electron microscopy with energy-dispersive X-ray spectroscopy (EDS). No spallation of the TBC was observed for pre-oxidised specimens of both alloys when exposed to air at 850 8C for 1100 cycles of 1 h dwell time at high temperature. SEM micrographs of the thermally grown oxide scale revealed outer mixed TiO 2 /Al 2 O 3 protrusions with a columnar structure. The protrusions contained small particles of zirconia and a low amount of about 0.5 at% zirconium was measured by EDS analysis throughout this outer oxide mixture. The TBCs exhibited excellent adherence on the oxide scale. Intercolumnar gaps and pores in the root area of the TBC were filled with titania and alumina. Below the outer columnar oxide scale, a broad porous zone of predominant titania was observed. The transition region between the oxide scale and substrate consisted of a discontinuous nitride layer intermixed with alumina particles and intermetallic phases rich in niobium formed at the nitride layer/substrate interface. When thermally cycled at 900 8C, the oxide scales on the alloy Ti-45Al-8Nb-0.2B-0.15C pre-oxidised in low partial pressure oxygen spalled off after 540 cycles. For the sample with TBC, spallation was observed after 810 cycles. Failure occurred in the thermally grown oxide near the oxide/nitride layer interface. Microstructural examinations revealed again oxide scales with columnar structure beneath the zirconia top coat and good adherence of the TBC on the thermally grown oxides formed at 900 8C.

Oxidation Behaviour of TBC Systems on γ-TiAl Based Alloy Ti–45Al–8Nb

“…7 and 11b). The morphology of the outer oxide scale seems to be influenced by the zirconia top coat, but the mechanism of formation of the structure with thin fine alumina columns embedded in titania is not yet fully understood [39][40][41]. The EB-PVD TBCs exhibited excellent adherence to the mixed titania and alumina oxide scale grown during high temperature exposure.…”

“…The EB-PVD TBCs exhibited excellent adherence to the mixed titania and alumina oxide scale grown during high temperature exposure. Failure of TBC systems on c-TiAl samples pre-oxidised or coated with an intermetallic Ti-Al-Cr layer was associated with spallation of the TGO scale, occurring very often near the oxide/nitride layer interface [38][39][40][41].…”

“…First applications of thermal protection on c-TiAl alloys yielded promising results [17,[37][38][39][40][41]. TBCs of yttria-stabilized zirconia produced by atmospheric plasma spraying and EB-PVD exhibited good adherence to the oxide scale grown on c-TiAl, pre-oxidised or coated with intermetallic protective layers.…”

Environmental protection of γ-TiAl based alloy Ti-45Al-8Nb by CrAlYN thin films and thermal barrier coatings

“…Due to the thermal insulation provided by ceramic topcoats with low thermal conductivities, internally cooled components can operate at increased temperatures. TBC systems on g-TiAl alloys exhibited promising performance when thermally cycled in air at temperatures between 850 and 1000 C [9,[28][29][30][31][32][33][34]. Failure observed was related to spallation of oxide scales formed on the substrate after degradation of the protective coatings.…”

Intermetallic Ti-Al-Cr Based Layers and Zirconia Topcoats Deposited on Gamma Titanium Aluminides for Environmental Protection

“…3a). These protrusions consisting of fine alumina particles in a string of beads-like array embedded in titania formed by outward diffusion of Ti and Al cations [8,9,11]. The inter-columnar gaps and pores of the TBC above the columnar scale were filled with titanium and aluminium oxides.…”